While this isn't the preferred method and probably not the easiest, if you get the cutting to grow roots and can successfully transplant them, it should work. This plant is usually very easy to propagate. There are two main disadvantages to growing cuttings in water. It's extra work to have to change the water out regularly and to have to plant the cuttings anyway when roots develop, but the real problem is that the roots that develop in water are much more brittle and much less able to function properly when planted in soil. It's easier to grow almost all types of cuttings in a breathable medium (potting mixes, pearlite, etc).

Planting your cuttings out soon will allow the the cutting to develop more well adapted roots and lesson the likelihood of rot.

An easy way to find the best way to propagate various types of plants is to google the scientific name (in this case, 'Hibiscus syriacus') along with the word 'propagation' or (in the cases you know which general method you want to use) 'seed', 'cutting', 'layering', or 'graft'.